Repeated attempts have been made to use the principles of nuclear magnetic resonance to log wells in oil exploration over the past several decades, with limited success. It was recognized that any particles of a formation having magnetic spin, for example atomic nuclei, protons, or electrons, have tendencies to align with a magnetic field which is imposed on the formation. Such a magnetic field may be naturally generated, as is the case with the earth's magnetic field B.sub.E which has an intensity of approximately 0.5 gauss in areas of the globe where boreholes are typically drilled. Any given particle in a formation is additionally influenced by localized magnetic fields associated with nearby magnetic particles, other paramagnetic materials, and the layer of ions which typically line pore walls of certain types of formations such as shales. These localized fields tend to be inhomogeneous, while the earth's magnetic field is relatively homogeneous.
The hydrogen nuclei (protons) of water and hydrocarbons occurring in rock pores produce NMR signals distinct from any signals induced in other rock constituents. A population of such nuclei, having a net magnetization, tends to align with any imposed field such as B.sub.E.
When a second magnetic field B.sub.1 transverse to B.sub.E is imposed on the protons by a logging tool electromagnet, the protons will align with the vector sum of B.sub.E and B.sub.1 after a sufficient polarization time t.sub.pol has passed. If the polarizing field B.sub.1 is then switched off, the protons will tend to precess about the B.sub.E vector with a characteristic Larmor frequency .omega..sub.L which depends on the strength of the earth's field B.sub.E and the gyromagnetic constant of the particle. Hydrogen nuclei precessing about a magnetic field B.sub.E of 0.5 gauss have a characteristic frequency of approximately 2 kHz. If a population of hydrogen nuclei were made to precess in phase, the combined magnetic fields of all the protons can generate a detectable oscillating voltage in a receiver coil. Since the magnetic moment of each proton produces field inhomogeneities, the precessing protons tend to lose their phase coherence over time, with a characteristic time constant called the transverse or spin-spin relaxation time T.sub.2. Furthermore, field inhomogeneities are also produced by other physical phenomena as mentioned above, so that the observed dephasing relaxation time T.sub.2 * is usually shorter than T.sub.2. Borehole magnetic resonance measurements of the above type are commercially available as a part of the NML service Schlumberger Technology Corporation, Houston, Texas ( Mark of Schlumberger). This tool is capable of measuring the Free Induction Decay of hydrogen nuclei in formation fluids, and to obtain the parameters T.sub.1 and T.sub.2 *. It does not measure the transverse relaxation time T.sub.2. A description of the basic components, operation and interpretation of the commercial logging tool used in the NML service is contained in a paper entitled, "An Improved Nuclear Magnetism Logging System and its Application to Formation Evaluation", by R. C. Herrick, S. H. Couturie and D. L. Best, presented at the 54th Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers (A.I.M.E., Dallas, Texas) in Las Vegas, Nevada, September 23-26, 1979; this paper, appended hereto, is incorporated herein by reference.
Other sequences of magnetic fields can be imposed on a population of protons in a formation, to measure other characteristics thereof. For example, if a pulse of alternating current having a frequency f is passed through a transmitter coil, producing an oscillating polarizing field B.sub.1 perpendicular to a static field B.sub.o, a population of protons precessing at a Larmor frequency equal to f would tend to align at an angle to B.sub.1. At the end of the pulse, when B.sub.1 is removed, the aligned protons experience a perpendicular torque, and precess about the B.sub.o vector. After a characteristic time called the longitudinal or spin-lattice relaxation time T.sub.1, the protons have relaxed to thermal equilibrium, wherein a weighted percentage of protons are aligned in the direction of B.sub.o. Various other sequences of imposed magnetic fields can be used, as is discussed in T. C. Farrar and E. D. Becker, "Pulse and Fourier Transform Nuclear Magnetic Resonance", Academic Press, N.Y. (1971), Chapter 2, pp. 18-33, which is incorporated herein by reference.
Although measurements of NMR characteristics of rock samples can be accurately made in a laboratory, making comparable measurements in a borehole is greatly exacerbated by the hostile environment where temperatures may reach several hundred degrees Fahrenheit, pressures reach thousands of p.s.i. and all of the equipment must be packed within a cylindrical volume of only several inches diameter.
One of the earliest NMR logging tools is shown in U.S. Pat. No. 3,289,072 granted November 29, 1966 to N. A. Schuster. A strong electromagnet is used to subject a sample of water or oil to a predetermined magnetic field. A RF coil produces an oscillating second magnetic field which causes nuclear magnetic resonance of protons in the sample and resonance of similar protons in the adjacent formation. Schuster proposed the use of a multipole electromagnet mounted in a wall engaging pad, or alternatively a larger electromagnet mounted within the logging sonde, to produce a static magnetic field B.sub.0. Schuster has also proposed other configurations of electromagnets and detection RF coils, for example in U.S. Pat. No. 3,083,335 granted on March 26, 1963, wherein the coil is positioned within a gap between two opposite poles of two bar magnets. Here, the magnetic field lines of the coil intersect field lines of the bar magnets perpendicularly, which is the optimum angle for inducing nuclear magnetic precession.
A more recent U.S. Pat. No. 3,667,035 granted May 30, 1972 to C. P. Slichter, shows a similar configuration of two coaxially aligned bar magnets and a RF coil positioned within the gap between opposite poles of the magnets. The term "bar magnet" is used herein to mean any magnet having only one north pole and one south pole, facing opposite directions, and may be either a permanent magnet or an electromagnet. Both the Slichter design and the Schuster design use electromagnets which require inconveniently large D.C. currents to be transmitted to a logging sonde through many thousands of feet of electrical cable.
U.S. Pat. No. 3,528,000 granted September 8, 1970 to H. F. Schwede shows one type of NMR logging tool in FIGS. 8 and 9, wherein a permanent magnet produces a first magnetic field which is fixed in its intensity, and an inductive coil produces an oscillating magnetic field whose frequency is varied over a selected range. Since the first magnetic field is produced by two opposite magnetic poles (one N and one S) placed side by side, the field is not homogeneous and the spatial gradient of the field is evidently non-zero at all points in the formation. In addition, since the first and second fields intersect not only in the formation, but also within the borehole, it is evident that protons constituting water or hydrocarbons within the borehole fluid contributes to signals detected by the RF coil, and must be removed either electronically or by chemically treating the borehole fluid, if a true formation measurement is desired.
Other NMR logging tools have been proposed which use permanent bar magnets, aligned coaxially in a logging sonde with a detection coil positioned in the gap between the magnets, for example as shown in U.S. Pat. No. 3,597,681 granted August 3, 1971 to W. B. Huckabay.
Another permanent magnet configuration has been proposed in U.S. Pat. No. 4,350,955 granted September 21, 1982 to J. A. Jackson, wherein two permanent bar magnets are coaxially aligned such that the RF detection coil is positioned in the gap between two similar poles of the two magnets. Similarly, United Kingdom Patent Application No. 2,141,236-A, published December 12, 1984, shows a similar configuration of coaxially aligned bar magnets with a detection coil positioned in a gap between the magnets. This type of configuration produces a toroidal region of homogeneous magnetic field wherein nuclear resonance may be measured. However, these tools may be adversely affected by signals from the borehole fluid in a large or deviated borehole where the tool would tend to lean against one side wall of the borehole. If the tool is designed to produce the toroidal region far away from the tool body, the produced magnetic field becomes much weaker, resulting in a significantly weaker signal. This configuration further requires that the detection coil or antenna be enclosed by a structure which would not block the oscillating electromagnetic waves of the measured signal. For example, fiberglass or some other non-metallic material is typically used; unfortunately, this structurally weakened link decreases the structural integrity of the tool and renders it considerably less useful in rough borehole conditions.
NMR measurement of particles other than hydrogen nuclei having magnetic spin have also been proposed. U.S. Pat. No. 3,439,260, granted April 15, 1968 to G. J. Benn et al, for example, discloses techniques of measuring magnetic resonance of carbon-13 nuclei in earth formations.
Other representative U.S. patents which have been granted for NMR logging tools and techniques include the following: 3,042,855 to R. J. S. Brown; 3,508,438 to R. P. Alger et al.; 3,483,465 to J. H. Baker, Jr.; 3,505,438 to R. P. Alger, et al.; 3,538,429 to J. H. Baker, Jr.; 4,035,718 to R. N. Chandler.
Each of the NMR logging tools which have been proposed or constructed has had practical deficiencies. All of them had to deal with the fundamental difficulties of making this kind of delicate measurements under severe conditions of temperature, pressure, and physical trauma typical of logging runs in oil wells. Furthermore, since the concentration of hydrogen nuclei within the borehole is much higher than the concentration in any rock formation, the undesirable NMR signals arising in a borehole are potentially much higher than any signals from surrounding formations. In order to alleviate this troubling phenomenon, it has been known in the art to treat the borehole fluid with a paramagnetic substance such as magnetite, and to circulate the treated fluid throughout the borehole before a logging run is made so that the relaxation time of hydrogen nuclei in the borehole is shortened by so much that its contribution to the NMR measurement is eliminated. Such pretreatment of borehole fluid is expensive and time consuming. Pretreatment may also introduce the same chemical, via the borehole, into adjacent permeable formations, and thus distort measurements.
It has also been recognized that those NMR logging tools which require powerful electromagnets tend to be unreliable because the high power currents flowing through the tool inevitably tend to break down various electronic components such as switches, especially under the high temperature environment in boreholes. The previous tools all required that the sonde or pad body be constructed of a non-metallic material such as fiberglass, synthetic rubber or teflon to enable detection of A.C. signals. These materials are considerably weaker than the alloy metals which are normally used in constructing other types of logging tools. The inability to use a strong metallic superstructure in constructing NMR logging tools has further contributed to their relative unpopularity in the industry.
Previous NMR logging tools typically required approximately 20-30 milliseconds, called "dead time", after a polarizing field pulse is shut and before the transmitting coil is sufficiently damped to permit measurements to be taken. During this dead time, considerable information of magnetic relaxation is irretrievably lost, and the S/N ratio is considerably degraded.
The commercially available NMR logging tool cannot directly measure the spin-spin relaxation time T.sub.2. Instead, the existing commercial tool obtains measures of the Free Fluid Index (FFI) and the observable dephasing relaxation time T.sub.2 *, also called the free induction decay time constant. Various log interpretation techniques may be used to derive other useful information as discussed in, e.g. "Applications of Nuclear Magnetism Logging to Formation Evaluation" by C. H. Neuman and R. J. S. Brown, Journal of Petroleum Technology (Dec. 1982) pp. 2853-2860, and in the Herrick et al. paper cited above.